Superconductivity refers to a state of materials in which the electrical resistance becomes zero when the material is cooled to a sufficiently low temperature, referred to as the critical temperature. One type of superconductor, referred to as a high temperature superconductor (HTS), has a critical temperature in excess of the boiling point of liquid nitrogen of 77° K at atmospheric pressure.
The use of superconductive materials and, in particular, superconducting cables, is advantageous because of the elimination of resistive losses. As a result, superconducting cables are being designed, built and tested for use in industrial applications, such as power transmission systems. While the lengths of superconducting cables used in power transmission systems have been relatively short while the merits of their use in this application is confirmed, such as on the order of less than a kilometer, it is anticipated that future applications will require much greater superconducting cable lengths, even in some cases in excess of 600 kilometers.
HTS cables are comprised of a core, such as a copper core, around which superconducting tape and wires are wrapped. A dielectric material surrounds the wires and a coolant, such as liquid nitrogen, flows through a vessel or tube over the dielectric providing both cooling and additional dielectric insulation for the conductor. This vessel or tube is then surrounded by an additional layer of thermal insulation (vacuum or other material) to minimize the rate of thermal input. In cold dielectric superconducting cables, the dielectric is on the inside of the area through which the coolant flows.
Conventionally, cryogenic refrigeration systems are used to cool cold dielectric superconducting cables to the critical temperature. Such systems contain a compressor for providing a refrigerant, or coolant, such as liquid nitrogen, at the critical temperature. The liquid nitrogen flows through an area disposed around the dielectric material, entering the area at one end of the cable and exiting the area at the opposite end of the cable to be returned to the cryogenic refrigeration system through a tube external to the superconducting cable assembly.
While this arrangement may be sufficient for cooling superconducting cables having short lengths on the order of up to 1 kilometer, cooling longer cable lengths requires additional cryogenic refrigeration systems because of the cooling capacity of such systems. Thus, many cryogenic refrigeration systems would be required along the length of the cable in order to maintain the entire cable length at the critical temperature. This arrangement becomes costly due to the cost of the additional refrigeration systems.